Method for producing fragmenting steel



United States Patent 3,513,038 METHOD FOR PROgUCIIlJYG FRAGMEN] ING TEERolf Weil, Hoboken, NJ., assignor to the United States Kt. America asrepresented by the Secretary of the rmy N0 Drawing. Filed Nov. 18, 1965,Ser. No. 508,606

Int. Cl. C21d 9/16 US. Cl. 148-49 4 Claims ABSTRACT OF THE DISCLOSUREThe invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment to me ofany royalty thereon.

This invention relates to fragmentation of ordnance articles. Moreparticularly, the invention relates to ordnance articles exhibitingimproved fragmentation properties and to a method for improvingfragmentation properties of forged steel ordnance articles.

The problem of fragmentation control in ordnance articles such asshells, bombs, grenades and the like is a significant one since itmaterially affects tactical use and, employment and substantiallyinfluences technical and economical aspects of production. Existence ofthe problem has not gone unnoticed and to date several methods ofsolution have been investigated while others are being considered. Thesemethods are based on controlling the fracture path by such mechanicaltechniques as machining grooves in the surface of the article or by suchmetallugrical techniques as using cast iron in which the graphitedistribution is determinative of fracture occurrence. While seemingly asolution to fragmentation control is offered by such methods, they areattended by collateral disadvantages, particularly in the area ofproduction and inability to withstand high velocity firing, which tendto offset and mitigate the benefits conferred.

Accordingly, a principal object of the present invention is to provide amethod for improving the fragmentation properties of ordnance articleswhich is unattended by the foregoing disadvantages of the prior art.

Another object of the invention is to provide a method for improving thefragmentation properties of forged steel ordnance articles.

Still another object of the invention is to provide a forged steelordnance article exhibiting improved fragmentation properties.

Another object of the invention is to provide a forged ordnance articleuseable at high velocities exhibiting improved fragmentation properties.

Other objects of the invention will in part be obvious and in partappear hereinafter in the following description of the invention and inthe appended claims.

The present invention solves the problem of fragmentation control inordnance articles, e.g., mortar sheels, by empolying forged steeltherein and involves the formation of a relatively brittle grainboundary network by proper heat treatment of such articles. Theformation of such network can be accomplished either in a steelinitially of hypereutectoid composition or in a hypoeutectoid steelcarburized to the hypereutectoid composition. Inthe latter case,improved fragmentation may be realized even by carburization of a partof the wall of an article to the hypereutectoid composition. Proper heattreatment involves the precipitation of carbide on the austenite grainboundaries by heating in or passing thru at suitable speeds theaustenite-iron carbide region of the iron-iron carbide equilibriumdiagram, quenching to form martensite from the remaining austenite, andtempering to stabilize the structure and dimensions, reach the desiredhardness level and relieve residual stresses.

Articles made in accordance with the disclosed invention exhibitfragmentation comparable to prior art articles made of malleable castiron and are attended by a better and wider range of mechanicalproperties. Additional advantages include reduced inspectionrequirements and shorter heat treating periods. As an economy factor,the latter looms significant when it is considered that the inventivemethod may require heat treating for a period of about four hours ascontrasted with the production of malleable cast iron which necessitatesheat treating white cast iron for about hours.

In the course of the investigation leading to the present invention thefollowing experimental procedure was conducted:

EXPERIMENTAL PROCEDURE Steel cylinders 1" outside diameter with a /a"wall and 2 /2" long were heat treated to obtain a large austenite grainsize with ferrite and cementite grain boundaries and a temperedmartensite matrix. This martensite matrix was to give the grains thetoughest possible structure. The large austentite grains were grown byheating the steel to various high austenitizing temperatures and holdingtherefor various times.

The ferrite network was obtained by slowly cooling samples of 1045 steelto 1350 F., a temperature at which some of the austenite, particularlythat in the grain boundary, transforms to ferrite. Subsequently thesteels were oil quenched to transform the remaining austenite tomartensite. The martensite was later tempered at 800 F. for one hour.

A carbide network in a hypereutectoid steel was obtained by a heattreatment similar to that which causes a ferrite network in ahypoeutectoid steel. However, as hypereutectoid steels have lowermelting points than those containing less carbon, it was necessary togrow large grains, which must be done at high austenitizing temperature,in a 1045 steel and later carburize at a lower temperature.

Cylinders of the same dimension as the steel ones were made of malleablecast iron for tests to compare them with 3 the steel of variousstructures. A forged 1045 untreated steel cylinder was also tested forcomparsion.

Three cylinders were heat treated simultaneously under any givencondition. One was examined metallographically and the other two werefragmented. The fragmentation test consisted of casting Composition Bcylinders, in diameter and long. The charge was then inserted in anassembly consisting of two brass cylinders, 1%" and A" long and of thesame inside and outside diameter as the test cylinders, which wereplaced below and above each iron or steel cylinder respectively in orderto eliminate end effects.

This assembly, consisting of the brass and steel'tubes and theComposition B cylinder, was placed in sawdust and the explosiveinitiated with a tetryl pellet. The fragments were collectedmagnetically and sorted according to size. Representative fragments fromeach size group of the various steels and cast irons tested wereexamined microscopically.

4 characteristics. A partial network, such as possessed by SpecimensD-2300-1 and D-2300-4, is considerably less effective and shows only aslight improvement over the forged, untreated steel. Of the two types ofintercrystalline networks, the carbide appears to result in betterfragmenting characteristics.

Metallographic examinations prior to fragmentation showed that the grainsize of the specimens are inhomogeneous and that each piece contained aconsiderable number of small crystallites.

Microscopic study of the fragments revealed that fracture occurred whennetwork was present practically always along the grain boundaries butnot necessarily along each grain boundary.

Fracture due to the hoop stress occurred readily regardless of thenetwork. However, with the possible exception of one cylinder possessinga complete ferrite network, only those samples with a complete or nearlycomplete carbide network broke into relatively equiaxed fragments TABLEI.-SUMMARIZED PROPERTIES OF FRACTURE SPECIMENS Steel Specimen Heattreatment Heated in dry hydrogen at 3,100 F. for 1% hrs. Cooled slowlyin hydrogen to 1,350 F. and held for 1 hr.

Quenched in oil. Tempered at 800 F. for an hour.

Heated at 2,250 F. in carbon block for 1% hrs. Trzgiisi ierred in blockto furnace at 1,700 F. with atmosphere. Cooled to 1,350 F. and held for1 hr.

Quenched in oil and tempered at 800 F. for 1 hr.

Heated at 2,300 F. in dry hydrogen for 1% hrs. Cooled slowly in hydrogento l,350 F. Quenched in oil.

Tempered at 800 F. for 1 hr.

Heated at 2,350 F. for 1% hrs. in carbon block, transferred to furnaceat 700 F. with RX atmosphere and cooled at 1,350 F. Held for 2 hrs.Quenched oil. Tempered at 800 F. for 1 hr.

Heated at 2,400 F. in dry hydrogen for 1 hr. Cooled to 1,350 F. and heldfor hr. Oil quenched. Tempered 1 hr. at 800 F.

ferred to packed carburizing box at 1,700 F Heated at 2,200 F. in dryhydrogen for 1% hrs. Transi held for 21 hrs. Cooled to 1350 F. and heldfor 1 hr.

Quenched in oil and tempered at 800 F. for 1 hr.

Heated in dry hydrogen to 2,300 F. for 2% hrs. Transferred tocarburizing box at 1,700 F. and held for 1% hrs. Quenched in oil.Tempered at 800 F. for 1 hr.

Packed in carburizing box with carburizing compound-Austlnitized at1,900 F. for 19 hrs. Brought down to 1,350 F. and held for 1 hr.Quenehed in oil.

Tempered for 1 hr. at 800 F.

Packed in carburizing box with carburizing c0111- pound-Austinitized at2,200 F. for 1% hrs. Brought down to 1,400 F. in furnace and held for 16hr.

Quenched in water. Tempered at 500 F. for hr.

Heated at 2,100 F. in dry hydrogen for 13/ hrs. Transterred to packedcarburizing box at 1,700 F. and held for 19 hrs. Cooled to 1,350 F. andheld for 1 hr.

Quenched in oil and tempered for 1 hr. at 800 F.

EXPERIMENTAL RESULTS Table I outlines the various heat treatments anddescribes the resulting microstructures and hardnesses. It can be notedthat steels with complete or nearly complete ferrite and carbidegrain-boundary networks and with partial networks were tested. Forcomparison, cast iron and untreated steel samples were also fragmented.The results of the fragmentation tests are summarized in Table II.(Table in C01. 5.)

DISCUSSION OF RESULTS The test results show that a complete or nearlycomplete grain-boundary network improves the fragmentation Grain dia.(average) cm.

Network continuity, percent Hardness N o.

Grain boundary Background structure Ferrite.

Tempered martinsete 0.199 .do do 29.4

0. Carbide--- 10 d0 0.125 do 10 d0 0.110 d0 95 do 0. do 100 do 0.232 .do95 do 42.5

tenite-iron carbide region of the iron-iron carbide equilibrium diagram,

holding said article in said temperature range until carbideprecipitation occur on the austenite grain boundaries,

quenching said article thereafter to form martensite,

tempering the quenched article to stabilize the resultant structure anddimensions and relieve residual stresses, said article being heated toabout 2100 F. in dry hydrogen for a period of about 1 /2 hours,

transferring said heated article to a packed carburizing box at about1700 F. and holding at this temperature for about 19 hours,

cooling said carburized article at about 1350 F. for a period of about 1hour, said cooled article being quenched in oil, and said quenchedarticle being tempered at about 800 F. for about 1 hour, said articleyielding a carbide grain boundary having about 95% network continuity.

References Cited UNITED STATES PATENTS 2,825,669 3/1958 Herzog 148-143OTHER REFERENCES A.S.M. Transactions, 1946, Preprint No. (pp. 18

and -39 CARL D. QUARFORTH, Primary Examiner A. J. STEINER, AssistantExaminer US. Cl. X.R.

